taoz said:
...what is the difference with dB+dB and dBm+dB?
When you see a decibel reference in the wild, there are TWO numbers to consider, the number of decibels, and
whatever reference it compares to. Thus the letter m after the dB refers to one milliwatt. That milliwatt could be at any impedance. Old signal generators used to refer to voltages at 50 ohms. Then someone figured out that these numbers made a hell of a lot more sense if you listed them in terms of dBm. Analog phone systems often derive dBm from RMS voltage measurements at 600 ohms.
When dealing with transmitters you sometimes see references to dBW (one watt). When dealing with antennas you often see gain references to an isotropic radiator (radiates equally in all directions). Among ham radio operators you often see reference to antenna gain compared to a dipole, which has a theoretical gain of 2.2 dB more than an isotropic radiator.
There are other references. I used to work on analog frequency division multiplex systems for phone trunks. There is dBrn (dB referenced to the noise floor), dBrnC (C-message wieghting on a 3400 Hz Channel referenced to the noise floor), dBrnC0 (this was dynamic range of the channel from the Test Level Point, or maximum power, to the noise floor with C-message weighting). You can get wild with all these references and they are commonly used in many system designs. And don't get me started about psophometric versus C-message weighting...
So when someone casually tosses the word decibel around your inner engineer should scream
Decibels over WHAT?
I taught this to my teen-age daughter and now she taunts her high school teachers with that question every time they discuss decibels. If she gets it, so can you.